1. Field of the Invention
The present disclosure is directed to processes for safe operation of a hydraulic hybrid vehicle system, and in particular, to processes for detecting and/or addressing safety conditions arising out of operation of the vehicle.
2. Description of the Related Art
Significant interest has been generated, in recent years, in hybrid vehicle technology as a way to improve fuel economy and reduce the environmental impact of the large number of vehicles in operation. The term hybrid is used in reference to vehicles employing two or more power sources to provide motive energy to the vehicle. For example, electric hybrid vehicles are currently available that employ an internal combustion engine and a generator which generates electricity that can be stored in a battery of storage cells. This stored energy is then used, as necessary, to drive an electric motor coupled to the drive train of the vehicle.
Hybrid vehicles may be grouped into two general classes, namely, parallel hybrid and series hybrid vehicles. Parallel hybrid vehicles are vehicles employing a more or less typical engine, transmission, and drive train, with additional components providing a second power path for the vehicle. According to one parallel hybrid scheme, the engine of a vehicle is used to generate surplus energy during periods when the vehicle is cruising at a steady speed, or otherwise demanding less than the engine is capable of providing when operating at its most efficient load. The surplus energy is then stored for future use.
It is known that internal combustion engines used in conventional motor vehicles are required to have a maximum output capacity that far exceeds the average requirements of the vehicle, inasmuch as such vehicles occasionally demand power output levels far exceeding the average power output, such as during acceleration from a stop, or for passing, etc. During these relatively brief periods of operation, much more power is required than during periods when the vehicle is cruising at a steady speed. Because of this requirement for a high level of available power, the engines in most conventional vehicles spend most of their time operating well below their most efficient speed and load.
By using excess capacity of the engine to produce energy that can be stored, the load on the engine can be increased to a point where the engine operates at a high level of fuel efficiency when in operation, while the excess energy is stored. The stored energy may then be used to enable engine-off operation, or to supplement the engine during periods when power requirements of the vehicle exceed the engine's maximum efficient output. Hybrid electric vehicles that are currently available generally operate according to the scheme broadly outlined above, utilizing a generator to add load to the engine and convert the excess power to electricity for storage in the battery, and later utilizing the battery and an electric motor to supplement the conventional drivetrain when more power to the wheels is required than can be efficiently produced by the engine alone.
There are other parallel hybrid vehicle configurations that have been proposed, that refine the basic system outlined above, or that provide some improved economy without departing significantly from the more conventional model. These other systems will not be discussed in detail here.
Series hybrid vehicles, in contrast to the parallel hybrid model, have no direct mechanical drivetrain between the engine and the drive wheels of the vehicle. They do not employ a drive shaft as described with reference to parallel hybrid vehicles. In a series hybrid vehicle, power from an engine is converted directly to a form that can be used by a secondary drive motor to power the vehicle, and that is also conducive to efficient storage. The engine can be operated at its most efficient load and speed without regard to variations in the speed of the vehicle. Depending on the capacity of the energy storage medium, a series hybrid vehicle may operate for extended periods with the engine shut down, operating on stored energy alone. Series hybrid vehicles are potentially more efficient than parallel hybrids because of the greater freedom to control engine operation for maximum efficiency, and because of the elimination of the mechanical drivetrain linking the engine to the wheels, thereby reducing the net weight of the vehicle, as compared to a parallel hybrid vehicle.
While electric hybrid vehicles have been briefly mentioned above, there is growing interest in the development of hydraulic hybrid vehicles, due to the potential for greater fuel economy, lower operating costs, and a lower environmental impact, as compared to electric hybrid vehicles. The greater fuel economy arises in part because of the relative superior efficiency of hydraulic systems in converting kinetic energy to a storable form, and in reconverting the stored energy potential to kinetic energy. The potential for lower operating costs is due to the fact that electric storage batteries currently available for use in hybrid vehicle operation are expensive and have a limited lifespan, with potential replacement at significant cost to the vehicle owner. The storage batteries are also an environmental concern because they contain large amounts of heavy metals that must be disposed of when the worn-out batteries are removed from the vehicles. Hydraulic systems such as might be employed in hybrid vehicles do not employ components that inherently require replacement, nor do they employ large amounts of toxic or harmful substances.
The configuration and operation of parallel and series hybrid vehicles are described in greater detail in the following references: U.S. Pat. No. 5,887,674, U.S. patent application Ser. No. 09/479,844, and U.S. patent application Ser. No. 10/386,029, all of which are incorporated herein by reference, in their entirety.